Prepare yogurt sauce. Stir together yogurt, lemon juice, mint and pepper. Refrigerate until ready to use.
Heat a grill or grill pan to medium heat (about 375-400°F).
Rub chicken thighs with harissa seasoning and oil.
Place Sear both sides of thigs for about 2 minutes per side over direct heat. Move to indirect heat or turn heat down and continue cooking until the thickest part of the thighs register at 165°F, about 8-10 minutes. Remove from grill.
Serve chicken thighs with prepared sauce and grilled onions (optional).
**If you use Greek style yogurt, thin it out with 1-2 tbsp cold water or milk.